In recent years, automatic record players, which incorporate an apparatus distinguishing between recorded and unrecorded portions of the disc have become well-known. The apparatus is convenient for operating the tone arms automatically and is needed for program-reproducing devices designed to select automatically predetermined recorded blocks to be played from the disc.
Such apparatus for record players must be able to distinguish differences between recorded blocks and unrecorded portions under the condition that the tone arm is above the disc surface. Further, it must reliably perform the detecting function, regardless of incident light.
Conventional apparatus takes advantage of the difference in light-reflecting efficiency between recorded blocks and unrecorded portions. The apparatus of the prior art for detecting between recorded and unrecorded portions of a disc is so constructed as to compare a signal detected from a light-sensitive device with a preset reference value. In such conventional apparatus, the detecting operation is apt to malfunction for any one of several reasons.
For one, the capacity of mass-produced light-emitting diodes and phototransistors to emit light and to detect light, respectively, is not uniform, and the performance of individual components is not consistent. For another, variations in external noise, such as room lighting, or variations of the incidence of the room lighting on the grooves or unrecorded portions, may affect the performance of the detecting apparatus, sometimes giving false signals as to recorded or unrecorded portions.
The lack of uniformity of light-emitting diodes and the phototransistors also adversely affects the production rate of mass-produced tone arms utilizing conventional apparatus for detecting the difference between recorded and unrecorded portions of the disc. The tone arms must be tested and adjusted one by one. In some cases the detection signal entirely fails to match the preset value.
Still further, conventional apparatus has the disadvantage that the apparatus can not clearly detect a boundary section between a recorded block and an unrecorded portion.